In the testing of lung capacity and the ability of a person to inhale or to exhale it is known to use a Venturi device to produce an air pressure differential, and to use an air pressure-to-electric voltage transducer to provide an electric signal which is a function of inhaled or exhaled air velocity. A computer integrates the velocity over a period of time to provide an indication of the actual volume of air inhaled or exhaled. Sensitive transducers are expensive, and efforts therefore have been made to utilize relatively inexpensive transducers. Such transducers of this nature are relatively insensitive, and produce a rather low level electric output.
This low level electric voltage has required high amplification, and this has caused electrical noise to be a problem.
There are times when a patient is unable to breathe for himself, and he is placed on a breathing apparatus providing forced inhalation. In such instance the exhalation is monitored, since this is done voluntarily by the patient, and is necessarily of the same volume as inhalation.
Exhalation is a non-linear function. Initial exhalation is at a relatively high rate. Exhalation then tapers off considerably, eventually reaching a very low level. There are generally time gaps from the end of inhalation to the start of exhalation, and from the end of exhalation to the start of inhalation. The most useful information is produced at the start of exhalation As exhalation tapers off the information becomes less valuable. There obviously is no useful respiratory information produced during the gaps in time at either end of the exhalation.
Noise signals are most obtrusive during the low level and no level periods of exhalation, since they may be on the same order of magnitude, or of greater magnitude than the signal from the transducer. On the other hand, noise signals are totally masked by the transducer signal during the initial period of rapid exhalation.